The present invention is based on an electrochemical sensor.
The sensors of this species must be heated in the active range to temperatures of more than ca. 350xc2x0 C. to achieve the necessary ionic conductivity of the solid electrolyte body. To increase the measuring accuracy of the sensor, it is known to control and, if necessary, to adjust the operating temperature of the measuring cell, i.e., of the solid electrolyte body in the measuring region. To this end, it is known to assign a heating device to the sensor, the heating device being capable of being switched on and off as a function of an operating temperature measured at the measuring cell.
To determine the operating temperature of the measuring cell, it is known to apply an a.c. voltage to the sensor and to use a measuring device to determine a total alternating-current resistance made up of the conjugate impedances of the solid electrolyte body and of the corresponding electrodes and electrode leads. The temperature-dependent internal resistance of the solid electrolyte body in the measuring region and, as such, its temperature in the measuring region can be deduced from the total resistance.
In the known method, it is disadvantageous that the measuring device, which determines the temperature-dependent resistance of the solid electrolyte body, uses a constant resistance of the electrodes and the electrode leads as a baseline. However, the resistance of the electrode leads and the electrodes is subject to a relatively high degree of scatter due to manufacture.
The measuring device adds this not insignificant scatter error to a temperature-dependent change in the resistance of the solid electrolyte body in the measuring region and provides a corresponding faulty control signal for the heating device of the sensor. As a result, the sensor is adjusted to an incorrect operating temperature.
It is further disadvantageous that, in the lead region, the solid electrolyte body forms an additional internal resistance that is connected in parallel to the internal resistance of the solid electrolyte body in the region of the electrodes (measuring region) and also makes a not insignificant contribution to the total resistance. If, in addition, the temperature in the lead region is higher than in the measuring region, the internal resistance of the solid electrolyte body in the lead region is reduced, and it makes a contribution to the total resistance that is dependent on the temperature of the solid electrolyte body in the lead region. As a result, the sensor is likewise adjusted to an incorrect operating temperature.
To avoid the effect of the internal resistance in the lead region, it is known from German Published Patent Application No. 198 37 607 to provide the lead of an electrode opposite the lead region of the solid electrolyte body with an electrically insulating layer. This design has the disadvantage that the use of at least one insulating layer additionally requires at least one printing step and is, therefore, expensive from a standpoint of production engineering.
In comparison with the related art, the electrochemical sensor according to the present invention has the advantage of an improved regulation of the operating temperature, thereby enabling the sensor to function more precisely and more uniformly.
An exemplary embodiment and/or exemplary method of the present invention provides that the internal resistance of a solid electrolyte body in a lead region between the electrode leads situated on the solid electrolyte body is significantly higher than the internal resistance of the solid electrolyte body in a measuring region between the corresponding electrodes. Thus, the contribution to the total resistance made by the internal resistance in the lead region of the solid electrolyte body, which is connected in parallel to the internal resistance in the measuring region of the solid electrolyte body, is significantly reduced. Thus, the influence of the internal resistance in the lead region on the temperature regulation may be negligible. Additionally, from a standpoint of production engineering, an electrically insulating layer may be dispensed with so that a printing step may no longer be required.
According to the present invention, the resistance of at least one electrode lead makes a small contribution to the total resistance. Furthermore, the electrode lead is made of a material having a smaller degree of processing scatter with respect to its resistance. Thus, the effect of the resistance of the electrode lead on the total resistance is smaller. The present invention additionally improves the regulation of the operating temperature of the sensor.
Designing the internal pump electrode lead and/or the reference electrode lead using a material having a lesser ionic conductivity or no ionic conductivity in comparison with the electrode in question has the additional advantage that the resistive coupling of the particular electrode leads that can lead to a loading effect of the pump voltage on the measuring voltage of the sensor cell is prevented. As a result, the lambda=1xe2x88x92ripple is decreased or even prevented, thereby further improving the control dynamic response of the sensor.
An additional advantage results from designing the external pump electrode lead and/or the internal pump electrode lead using a material having a low resistance in comparison with the material of the electrode in question. As a result, the drop in the pump voltage in the external pump electrode lead and/or internal pump electrode is reduced, thereby improving pump function.
A particular embodiment of the present invention provides that the reference electrode lead is situated in the layer plane of the heater, thereby eliminating at least one printing step. In a further embodiment of the present invention, the heater and reference electrode lead are produced from the same material, thereby resulting in an additional advantage from a standpoint of production engineering.